1. Field of the Invention
This invention relates to a ball bearing device for the swing arm of a disc driving apparatus for a magnetic disc, an optical disc or the like.
2. Related Background Art
The bearing portion of a ball bearing device for a swing arm according to the prior art, as shown in FIG. 12A of the accompanying drawings, has ball bearings 1 and 11 having grease enclosed therein, and the ball bearings are used under a pre-load at predetermined positions. FIG. 12B of the accompanying drawings shows the surface pressure distribution 14 of the ball bearing 11 in a cross-section taken along the line 12Bxe2x80x9412B of FIG. 12A, and the surface pressure distribution 14 is indicated by a circle, and the radial bearing rigidity of all portions in the circumferential direction thereof is constant.
As shown, there are fitting portions on both of the inner diametral surface of the inner race of the ball bearing fitted to a shaft 5 and the outer diametral surface of the outer race fitted to a housing 10.
These fitting portions are adhesively secured to the shaft 5 and the housing 10 over the entire bearing circumference of the ball bearings 1 and 11.
FIG. 12C of the accompanying drawings schematically shows the actually used state of the ball bearing device for a swing arm according to the prior art.
Recently, higher and higher density has been required of magnetic disc apparatuses. Therefore, the width of tracks for recording signals on the disc has become narrower and narrower, and higher speed of access to a target track and higher accuracy of positioning performance have been required of a swing arm carrying a head for recording and reproducing signals.
In order to satisfy the required higher speed and higher accuracy of control, freedom from a torque fluctuation such as a torque spike, a low torque, etc. is required of the ball bearings 1 and 11 supporting the swing arm 9. Also, high stability and low torque of the control of the swing arm system when controlled at a high speed are desired.
Heretofore, in the ball bearing device for the swing arm, as shown in FIG. 12A, the two ball bearings 1 and 11 have been accurately assembled together with a pre-load applied in the axial direction thereof to thereby achieve the higher accuracy of the bearing device. The present invention has as its object to provide a ball bearing device for a swing arm in which the stability of a swing arm system is enhanced and the higher speed and higher accuracy of control are possible.
In the present invention, in order to enhance the stability of the swing arm and the swing arm system and achieve higher speed and higher accuracy of control of the swing arm, attention has been paid to the rigidity of the ball bearing device in the radial direction thereof. It has been confirmed that it is effective to vary the rigidity in the radial direction in accordance with the following methods.
1) During the operation of the ball bearing device, the locus of the displacement of the centers of the balls is made elliptical, whereby the rigidity of the ball bearing device in the radial direction thereof is made to differ between two radial directions that are orthogonal to each other.
Particularly in the case of a straight swing arm, it is preferable that the rigidity in a first radial direction parallel to the axis of the arm be made great relative to the rigidity in a radial direction orthogonal to the first direction. Further it is preferable in enhancing the stability of the control of the swing arm to make the rigidity in the first direction greater by 5% or more.
2) The rigidity of the ball bearing device in the radial direction thereof is greatly reduced as compared with the rigidity of the ball bearing device of the prior-art construction in the radial direction thereof.
It is particularly preferable in enhancing the stability of the control of the swing arm that the rigidity in the radial direction be reduced to 50% or less relative to the rigidity of the ball bearing device of the prior-art construction in the radial direction thereof.
According to one aspect of the present invention, the ball bearing has a difference between the radial bearing rigidities in two directions orthogonal to each other.
In the present invention, the locus of the displacement of the centers of the balls of the ball bearing can be made elliptical, whereby the amount of elastic deformation of the balls and raceway surface is made to differ between the major axis side and the minor axis side of the ellipse. As a result, the contact surface pressure differs between the major axis direction and the minor axis direction of the ellipse and the radial bearing rigidity differs. Or further, if the radial bearing gap in a second radial direction is made larger than the radial bearing gap in a first radial direction orthogonal to the second radial direction, the balls and the raceway surface do not contact with each other in the second radial direction, and the radial bearing rigidity in that radial direction can be made small as compared with the radial bearing rigidity in the first radial direction. As a result, the radial bearing rigidities of the ball bearing in the two directions orthogonal to each other can be made to have a difference therebetween.
Specifically, in a ball bearing according to the present invention, it is preferable that balls be disposed between one raceway surface provided on an inner member and another raceway surface provided on an outer member and that at least one raceway surface be elliptical in its cross-section in a radial plane. Or further, it is preferable that the radial bearing gap in a first radial direction be smaller than the radial bearing gap orthogonal to the first radial direction. For example, in the ball bearing according to the present invention, the inner member may fit to a shaft and in the first radial direction, the inner member may be pressed into the shaft, and in the second radial direction, a gap may be provided between the inner member and the shaft. Or in a ball bearing according to the present invention, the outer member may fit to a housing and in the first radial direction, the outer member may be pressed into the housing, and in the second radial direction, a gap may be provided between the outer member and the housing.
Or further, in a ball bearing according to the present invention, the inner member may fit to the shaft and the outer member may fit to the housing, and a plane containing the centers of a plurality of balls disposed between one raceway surface of the inner member and the other raceway surface of the outer member may be inclined with respect to a plane perpendicular to the center axis of the rocking member of one of the shaft and the housing.
According to an aspect of the present invention, the ball bearing is characterized in that at least one of the axial length of the portion fitting to the shaft at the inner diametral surface of an inner race and the axial length of the portion fitting to the housing at the outer diametral surface of an outer race is xc2xd or less of the bearing width of the ball bearing. Thereby, the rigidity of the ball bearing device in the radial direction thereof is greatly reduced relative to the rigidity of the ball bearing device of the prior-art construction in the radial direction thereof.
According to another aspect of the present invention, the ball bearing device is characterized in that the radial bearing rigidities in two directions orthogonal to each other have a difference therebetween and at least one of the axial length of the portion fitted to the shaft at the inner diametral surface of the inner race and the axial length of the portion fitted to the housing at the outer diametral surface of the outer race is xc2xd or less of the bearing width of the ball bearing.
Other modes of the present invention will become apparent from the embodiments of the invention shown and described herein.